The present invention relates to the field of co-fired glass and/or ceramic (hereafter just ceramic) structures and, more particularly, to techniques for processing such structures so as to be able to form adherent layers of metallurgy on their surface.
Ceramic structures, usually and preferably multilayered, are used in the production of electronic substrates and devices. Many different types of structures can be used, and a few of these structures are described below. For example, a multilayered ceramic circuit substrate may comprise patterned metal layers which act as electrical conductors sandwiched between ceramic layers which act as insulators. The substrates may be designed with termination pads for attaching semiconductor chips, connector leads, capacitors, resistors, covers, etc. Interconnection between buried conductor levels can be achieved through vias formed by metal paste-filled holes in the individual glass ceramic layers formed prior to lamination, which, upon sintering, will become a sintered dense metal interconnection of metal-based conductor.
In general, conventional ceramic structures are formed from ceramic greensheets which are prepared by mixing a ceramic particulate, a catalyst (e.g., such as that disclosed in Herron, et al., U.S. Pat. No. 4,627,160), a thermoplastic polymeric binder, plasticizers and solvents. This composition is spread or cast into ceramic sheets or slips from which the solvents are subsequently volatilized to provide coherent and self-supporting flexible green sheets. After blanking, stacking and laminating, the green sheets are eventually fired at temperatures sufficient to drive off the polymeric binder resin and sinter the ceramic particulates together into a densified ceramic substrate.
The electrical conductors used in formation of the electronic substrate may be high melting point metals such as molybdenum and tungsten or a noble metal such as gold. However, it is more desirable to use a conductor having a low electrical resistance and low cost, such as copper and alloys thereof.
Use of copper-based conductors in the multilayered structures requires the use of process techniques which do not oxidize the copper during the removal of binder resins and solvents, and sintering of the ceramic particulates together into the densified ceramic substrate.
For example, a typical firing cycle consists of pyrolyzing the binder, burning the binder off in a steam ambient, typically water vapor plus hydrogen, and then replacing the steam ambient with an inert (neutral) ambient such as nitrogen and sintering the structure to its final densified state, followed by a cool down, again in an inert atmosphere such as nitrogen.
This seemingly simple firing cycle is, in fact, extraordinarily complex in nature and has taken years and large expense to achieve. It is not an understatement to say that improvements in this art come in small steps rather than in great leaps.
It would be desirable to be able to bond a copper or gold layer to the surface of the ceramic structure. Such a metal layer might serve as a capture pad, bonding pad, input/output (I/O) pad, wiring line or other use. A pervasive problem, however, is the bonding of this metal layer to the ceramic structure without oxidizing the copper patterns. Copper and gold are notoriously deficient in adhering to ceramic structures under these kinds of sintering conditions. Obviously, an insufficiently bonded metal layer would suffer delamination during the bonding process or in use.
The art is replete with numerous metallurgies and techniques utilized to achieve an adherent metal layer on a ceramic surface.
Flaitz et al. U.S. Pat. No. 4,764,341 discloses the bonding of, for example, a nickel layer to an oxide ceramic by interposing a ternary oxide of the metal to be joined which, for this example, is NiAl.sub.2 O.sub.4. The firing cycle is adjusted so that the nickel layer does not form nickel oxide and yet it is sufficiently oxidative to ensure the removal of carbon residue from the polymeric binder. The end result is nickel bonded to the NiAl.sub.2 O.sub.4 which, in turn, is bonded to the oxide ceramic.
Chance et al. U.S. patent application Ser. No. 929975, filed Nov. 12, 1986, "Method For Producing High Density Multilayered Glass-Ceramic Structures With Metallic Based Conductors", now abandoned but published in Japan on May 12, 1988, as J89050120-B, discloses the bonding of nickel pads to a glass-ceramic material during the firing cycle. A key element of the process is the oxidizing of the nickel pad during the crystallization segment of the firing cycle. The NiO film causes the nickel pad to bond to the glass-ceramic material. Thereafter, the NiO film may be removed by chemical means or by a reducing ambient. This latter process step, however, risks reducing the NiO bonds the nickel pad to the glass-ceramic material with the consequence that delamination of the nickel pads may occur.
Nakatani et al. U.S. Pat. No. 4,863,683 discloses a glass-ceramic substrate having an oxide paste, e.g., NiO, which during the firing cycle is reduced to the base metal, nickel in this case. The remainder of the firing cycle is completed without oxidizing the base metal.
deBruin et al. U.S. Pat. No. 4,050,956 discloses the bonding of certain metals, including copper, nickel and gold, to a refractory oxide ceramic wherein the metal and ceramic are placed in an abutting relationship and then fired in air. Ebata et al. U.S. Pat. No. 4,631,099 similarly discloses joining copper or a copper alloy to an oxide ceramic by placing them in abutting contact and then firing in an oxidative atmosphere.
Larry U.S. Pat. No. 3,854,957 discloses a metallizing paste consisting of noble metals and NiO applied to a ceramic substrate. The firing atmosphere is not specified. The NiO is added to increase the adhesion of the paste to the ceramic substrate.
The disclosure of all of the previous references are incorporated by reference herein.
Notwithstanding the prior art, there still remains a need to bond a metal layer to a ceramic structure under conditions which are non-oxidizing to the copper patterns in the ceramic structure.
Accordingly, it is an object of the invention to form an adherent layer of metal on a ceramic structure without causing oxidation of the internal copper patterns.
It is another object of the invention to form a copper, nickel or gold layer on the surface of a ceramic structure without causing oxidation of the internal copper patterns.
It is yet another object of the present invention to form such a copper, nickel or gold layer which is adherent and will not delaminate during bonding processes or in use.